1. Field of the Invention
This invention pertains to a low temperature (approximately 700.degree. F. head temperature) Stirling engine driven by a solar powered apparatus.
2. Description of the Prior Art
The societal need for renewable, non-polluting sources of energy, particularly electrical energy suitable for distribution by conventional power lines, has become obvious in the face of increased environmental concerns regarding global warming, acid rain and nuclear fuel disposal and increased economic concerns regarding the high cost of energy production, not to mention the long lead times required to build a conventional large-scale power plant in the face of mounting regulatory obstacles.
While small-scale production of electricity by renewable means (such as solar) is desirable in many respects, fundamental thermodynamic constraints on the attainable energy conversion efficiency have tended to make small-scale low-temperature heat source applications impractical. However, high-temperature heat source applications have been difficult to maintain, particularly if the thermal transfer fluid was a liquid metal such as sodium or potassium which, while having the distinctive thermal advantage of a low Prandtl number, is highly reactive with water and has a tendency to leach alloying materials from the hot loop surfaces and deposit them at the cold loop surfaces.
More particularly, in solar applications, the use of a high heater head temperature (typically 1300.degree. F. in the past) to achieve high cycle efficiency has necessitated a small aperture solar energy receiver (to minimize aperture radiation losses) and, consequently, a collector with a high concentration ratio and very accurate contour control; a heat-pipe-type thermal transport subsystem to avoid hot spots on the already high temperature head; the use of liquid metal heat pipe transfer media (typically potassium-sodium mixtures) to achieve proper heat-pipe-type operation at the temperature of interest; superalloy materials in the engine-alternator heater head (due to concerns of the creep strength of the material at the operating temperature, its yield strength at both high and low temperatures, and its ability to resist high and low cycle fatigue); and the use of helium rather than hydrogen as a Stirling-cycle working fluid due to the high-temperature permeation of hydrogen through the heater head into the heat pipe system. These requirements adversely impacted the manufacturing cost and performance of the system.
Moreover, a substantial drawback to solar-to-electrical energy conversion has been the unavailability of sunlight during either nighttime or inclement weather.